1. Field of the Invention
The present invention relates to a focus adjustment apparatus and an image capturing apparatus, in particular, to a focus adjustment apparatus and an image capturing apparatus capable of rapidly bringing subjects into focus for image capture.
2. Description of the Related Art
As a focusing technique used for film cameras and digital cameras, there is a focus adjustment technique using evaluation functions, such as point spread functions respectively corresponding to focus positions of a lens, and a database of characteristic values of defocus to calculate the focus position relative to a subject image. Specifically, in Japanese Patent Laid-Open No. 06-181532, point spread function data corresponding to focus positions are databased in advance. Subject images in which the amounts of defocus are uncertain are actually obtained, data on the subject images is Fourier transformed, and data obtained by dividing the Fourier transformed data by the point spread functions is inversely Fourier transformed to obtain restored images. Then the focus position corresponding to the point spread function which produces the smallest error value of the restored images obtained by the Fourier transformation, is determined as its focus position relative to a real image. Then, the lens is moved to a desired position based on the determined focus position. However, when carrying out a higher-accuracy focus adjustment by using such a technique, it is needed to prepare a large number of point spread functions in advance corresponding to slight differences in the lens positions. And further, it is also needed to make vast quantities of calculations at times of divisions by point spread functions, inverse Fourier transformations, and extractions of error values, and therefore there is also a possibility that it takes much time to adjust the focus of the lens.
Japanese Patent Laid-Open No. 2000-152064 has proposed a method for overcoming a delay in focus adjustment due to such problems, i.e., an increase in storage capacity and calculation quantity for provision of many point spread functions. This document describes adoption of a technique of combining rough control for movement to an in-focus position using point spread functions and fine control by a hill-climbing detection method or a technique of determining the infinite-distance focus position or minimum-distance focus position as its initial focus position to limit the number of its determinable focus positions.